Eccentric Sand Dollar, Dendraster excentricus
The Eccentric Sand Dollar (Dendraster excentricus) known also as the Sea-Cake, Biscuit-Urchin, Western Sand Dollar, or the Pacific Sand Dollar, is a member of the order Clypeasteroida, better known as sand dollars, a species of flattened, burrowing sea urchins located along the Pacific Ocean from Alaska to Baja California.
This species is an irregular echinoid that is flattened and burrows into the sand, unlike the regular echinoids, or sea urchins. It can be found living within the Pacific Ocean from Alaska to Baja California. The range for this species is larger and includes the range of the other two existing species of D. vizcainoensis and D. terminalis. The flower pattern seen in this species is off-center, giving it the species name excentricus.
The coloration regarding this species is gray, black, brown, or shades of purple. Their size varies up to 100 millimeters wide, and a dome shaped carapace varying in height to about ten millimeters, with a circular body or test. The body is covered with fine and spiny tube like feet with cilia, and like other echinoderms, they have five-fold radial symmetry. The mouth, anus, and food grooves are on the lower surface and the aboral surface has a petalidium, or petal shaped structure, with tube feet. Some non-living individuals have a gray/white test, or skeleton, which is frequently found washed up on beaches. It has a water-vascular system from the internal cavity or coelom that connects the tube feet. The tube feet are positioned in five paired rows and are found on the ambulacra–the five radial areas on the undersurface of the animal, and are utilized for movement, respiration, and feeding. The spine are usually club-shaped in the adults, and less so in the juveniles. At the center on the aboral side, is the madreporite, which is a perforated plate-liked structure, and on the interambulacra are the four tiny genital pores. Radiating out from the genital pores are the five flower petals, which represent the ambulacral radii. The mouth is in the center on the bottom side, with the anus towards the edge.
They’re either found sub tidally in bays or on open coastal areas or in the low intertidal zone on the Northeast Pacific coast. It can sustain depths of up to 40 to 90 meters, but is usually found in more shallow areas. Sand dollars are normally crowded together over an area half buried within the sand. As many as 625 sand dollars are able to live in one square yard. It’s the only sand dollar found in Oregon and Washington. It’s also been found on Burfoot Beach in the South Puget Sound.
Similar to its cousins, dendraster is a suspension feeder which feeds on crustacean larvae, small copepods, plankton, diatoms, and detritus. Adult sand dollars move mostly by waving their spines, while juveniles use their tube feet. The tube feet along the petalidium are larger in size and are utilized for respiration while tube feet elsewhere on the body are smaller in size and are used for feeding and movement. They often move around if they’re lying flat. When feeding, they normally lie at an angle with their anterior end buried and catch small prey and algae with its pedicellariae, tube feet, and spines and pass them to the mouth. Their mouth includes a small Aristotle’s lantern structure found in most Echinoids. In high currents, the adults grow heavier skeletons while the juveniles swallow heavy sand grains to keep from being swept away. When they are being preyed upon, they will bury themselves.
The sexes are separate. There are no obvious differences in extern features of the two sexes. The act of reproduction is sexual. D. excentricus reaches sexual maturity between one and four years, spawning in late spring and early summer. Fertilization is external, the female discharges the eggs though her gonopores and they are fertilized by the male, who protrudes his genital papilla from his body wall. This is one reason why they’re believed to live in large groups and tend to release gametes at the same time into the water column. The eggs are pale orange and are covered by a thick jelly like coat which keeps adults from eating the eggs.
The first larval stage is referred to as a prism. After this stage, the embryo will develop two arms transforming itself into an echinopluteus larva. This is followed by the development of arms, until it attains eight arms altogether. After this, the larva develops an echinus or juvenile rudiment, which will become the juvenile. The nektonic larvae are pelagic and travel away from the group of parents with the current. The developed larvae will receive a chemical cue from adults to settle down into a bed of sand dollars and begin to undergo metamorphosis to their adult sand dollar form. As adults, they are benthos and remain on the sandy bottom.
Image Caption: Eccentric Sand Dollar (Dendraster excentricus), Ganges Harbour, Salt Spring Island, British Columbia. Credit: D. Gordon E. Robertson/Wikipedia (CC BY-SA 3.0)